Alumina-boron carbide ceramics and methods of making and using the same

ABSTRACT

A ceramic body that contains between about 15 volume percent and about 35 volume percent of a boron carbide irregular-shaped phase and at least about 50 volume percent of alumina. The substrate has a fracture toughness (K IC , 18.5 Kg Load E&amp;C) greater than or equal to about 4.5 MPa·m 0.5 .

BACKGROUND OF THE INVENTION

The disclosure of the present patent application pertains to a ceramicbody that contains alumina and boron carbide, as well as a method ofmaking the same and a method of using the same. More specifically, thedisclosure of the present patent application pertains to a ceramic body(for use as a ceramic cutting insert or a substrate for a coated ceramiccutting insert or a ceramic wear part) that contains alumina and a boroncarbide phase, as well as a method (e.g., hot pressing) of making thesame and a method of using the same.

Ceramic materials have been used as cutting inserts and as wear membersfor a number of years. These ceramic materials include silicon nitrideor silicon nitride-based ceramics, SiAlON or SiAlON-based ceramics, andalumina or alumina-based ceramics. One of the first ceramic cuttinginserts was an alumina cutting insert. See Dörre et al., “Alumina,Processing, Properties, and Applications”, Springer-Verlag (1984), pages254-265. The alumina cutting insert was essentially over 99.7 percentalumina. Later on, the alumina ceramic was modified by the addition oftitanium carbide. See Whitney, “Modern Ceramic Cutting Tool Materials”,Presentation at October, 1982 ASM Metals Congress in St. Louis, Mo.

Over the passage of time, there have been a number of other additivesused in conjunction with alumina to form an alumina-based ceramiccutting insert. Examples of the additives include the use of siliconcarbide whiskers such as the ceramics that appear to be disclosed in theU.S. Pat. No. 4,789,277 to Rhodes et al. and U.S. Pat. No. 4,961,757 toRhodes et al. In an alumina-SiC whisker ceramic, the Rhodes et al.patents appears to show that the (K_(IC)) fracture toughness increased(4.15 to 8.9 MPa·m^(0.5)) as the SiC whisker content increased from 0 to24 volume percent. The Rhodes et al. patents then appear to show thatthe fracture toughness decreased (8.9 to 7.6 MPa·m^(0.5)) as the SiCwhisker content increased from 24 to 35 volume percent. European PatentNo. 0 335 602 B1 to Lauder appears to disclose the use of siliconcarbide whiskers in alumina along with the addition of additives likezirconia, yttria, hafnia, magnesia, lanthana or other rare earth oxides,silicon nitride, titanium carbide, titanium nitride or mixtures thereof.The use of silicon carbide whiskers along with alumina is described inBillman et al., “Machining with Al₂O₃—SiC Whisker Cutting Tools”,Ceramic Bulletin, Vol. 67, No. 6 (1988) pages 1016-1019. U.S. Pat. No.4,343,909 to Adams et al. appears to disclose the use of zirconia andtitanium diboride along with alumina (and a sintering aid). U.S. Pat.No. 4,543,343 to Iyori et al. discloses the use of titanium boride andzirconia along with alumina.

In the article written by Liu and Ownby (Liu et al. entitled “PhysicalProperties of Alumina-Boron Carbide Whisker/Particle Composites” CeramicEng. Sci. Proc. 12 (7-8) pp. 1245-1253 (1991) there is a disclosure of aceramic comprising alumina and boron carbide particles. In this regard,the Liu et al. composites appear to disclose alumina (A16SG fromAlcoa)-boron carbide particle (0.2 to 7 μm particles size) compositesalong with boron carbide that is present in amounts of 5.0, 10.0, 15.0and 20.0 volume percent (the balance equals alumina). The examples wereeither sintered at 1500° C. or 1600° C. for 3 hours or hot-pressed underthe hot pressing parameters that comprised a temperature equal to 1520°C. for a duration equal to 20 minutes. The sintered composites had adensity less than 80 percent of the theoretical density. The hot pressedceramics had a density of greater than 98 percent of the theoreticaldensity. The hot pressing pressure seems to be absent from thedisclosures of this Liu et al. article.

This Liu et al. article appears to show that the fracture toughness(measured by the Chevron Notched Short Rod (CNSR) technique) improvesfrom 0 volume percent boron carbide particles to 5.0 volume percentboron carbide particles wherein the fracture toughness of the 5.0 volumepercent boron carbide particle-alumina ceramic equals about 5.2MPa·m^(0.5). However, the fracture toughness drops off at boron carbideparticle contents greater than 5.0 volume percent. More specifically,the fracture toughness diminishes at boron carbide particle contents of10.0, 15.0 and 20.0 volume percent. The fracture toughness of the 20.0volume percent boron carbide particle-alumina ceramic appears to equalabout 4.5 MPa·m^(0.5). Liu et al also shows that the flexural strengthimproves from 0 volume percent boron carbide particles to 5.0 volumepercent boron carbide particles. The 5.0 volume percent boron carbideparticle-alumina material has a flexural strength equal to about 575MPa. The flexural strength levels off (i.e., remains essentially thesame) at boron carbide particle contents greater than 5.0 volume percent(i.e., boron carbide particle contents of 10.0, 15.0 and 20.0 volumepercent). The 20.0 volume percent boron carbide particle-aluminamaterial has a flexural strength equal to about 590 MPa.

In the article (1991—American Institute of Physics) written by Liu etal. entitled “Boron Containing Ceramic Particulate and WhiskerEnhancement of the Fracture Toughness of Ceramic Matrix Composites”there is a disclosure of a ceramic comprising alumina and boron carbideparticles. These Liu et al composites appear to disclose α-alumina-boroncarbide particle composites wherein the boron carbide is present inamounts of 5.0, 10.0, 15.0 and 20.0 volume percent (the balance equalsalumina). The examples were hot-pressed under the hot pressingparameters that comprised a temperature equal to 1480° C. so that theceramic had a density of greater than 98 percent of the theoreticaldensity. The hot pressing duration and the hot pressing pressure appearto be absent from the disclosure of this Liu et al. article.

The Liu et al. articles show that the fracture toughness (CNSRtechnique) improves from 0 volume percent boron carbide particles to 5.0volume percent boron carbide particles wherein the fracture toughness ofthe 5.0 volume percent boron carbide particle-alumina ceramic equalsabout 5.5 MPa·m^(0.5). However, the fracture toughness drops off atboron carbide particle contents greater than 5.0 volume percent. Morespecifically, the fracture toughness diminishes at boron carbideparticle contents of 10.0, 15.0 and 20.0 volume percent. The fracturetoughness of the 20.0 volume percent boron carbide particle-aluminaceramic appears to equal about 4.6 MPa·m^(0.5).

In the article written by Liu et al. entitled “Boron Carbide ReinforcedAlumina Composites” Journal American Ceramic Society 74 (3) pp. 674-677(1991)) there is a disclosure of a ceramic comprising alumina and boroncarbide particles. The Liu et al. composites appear to disclose fineα-alumina (A16SG from Alcoa)-boron carbide “shard like” particle (0.2 to7 μm particles size) composites along with boron carbide that is presentin amounts of 5.0, 10.0, 15.0 and 20.0 volume percent (the balanceequals alumina). The examples were hot-pressed under the hot pressingparameters that comprised a temperature equal to 1520° C. for durationequal to 20 minutes so that the ceramic had a density of greater than 98percent of the theoretical density. The hot pressing pressure seems tobe absent from the disclosures of the Liu et al. articles.

This Liu et al. article appears to show that the fracture toughness(CNSR technique) improves from 0 volume percent boron carbide particlesto 5.0 volume percent boron carbide particles wherein the fracturetoughness of the 5.0 volume percent boron carbide particle-aluminaceramic equals about 5.3 MPa·m^(0.5). However, the fracture toughnessdrops off at boron carbide particle contents greater than 5.0 volumepercent. More specifically, the fracture toughness diminishes at boroncarbide particle contents of 10.0, 15.0 and 20.0 volume percent. Thefracture toughness of the 20.0 volume percent boron carbideparticle-alumina ceramic appears to equal about 4.6 MPa·m^(0.5). Liu etal also shows that the flexural strength improves from 0 volume percentboron carbide particles to 5.0 volume percent boron carbide particles.The 5.0 volume percent boron carbide particle-alumina material has aflexural strength equal to about 580 MPa. The flexural strength levelsoff (i.e., remains essentially the same) at boron carbide particlecontents greater than 5.0 volume percent (i.e., boron carbide particlecontents of 10.0, 15.0 and 20.0 volume percent). The 20.0 volume percentboron carbide particle-alumina material has a flexural strength equal toabout 600 MPa.

The Jung and Kim article entitled “Sintering and Characterization ofAl₂O₃—B₄C composites”, Journal of Material Science 26 (1991) pp.5037-5040 concerns the sintering of alumina-boron carbide composites.According to the article, for composites sintered at 1850° for 60minutes the density was about 97 percent for a boron carbide contentthat ranged between 5 to 20 volume percent boron carbide. According tothe Jung et al. article, the flexural strength had a maximum value of550 MPa for an alumina-20 volume percent boron carbide composite thathad been sintered at 1850° for 60 minutes. According to the Jung et al.article, for a composite sintered at 1850° for 60 minutes. The Vickersmicro-hardness increased with increasing boron carbide content to 30volume percent. For this same composite, the fracture toughness slightlyincreases with increasing boron carbide contents up to 20 volumepercent. The maximum fracture toughness is 4 MPa·m_(1/2).

Air Force Report AFML-TR-69-50 by E. Dow Whitney entitled “New andImproved Cutting Tool Materials” (1969) discloses an alumina-boroncarbide composite. At page 119, the Report reads:

-   -   The metal carbides, WC, TaC, TiC, B₄C and SiC were selected as        additives for improving the general properties of hot presses        alumina. Mixtures of Al₂O₃ containing 1.25 wt. % of each        additive were hot pressed at 1600° C., 2600 psi, for 30 minutes        in a nitrogen atmosphere. In FIGS. 147 to 149 are shown the        heating densification curves of these systems. Density increased        rapidly from about 1200° C. and reached almost 100% relative        density at temperatures below 1600° C.        Table 52 of the Air Force Report appears to show that the        addition of 1.25 weight percent boron carbide to alumina        increased the MOR from 30,700 psi (for alumina) to 42,500 psi        (alumina+1.25 weight percent boron carbide), but the hardness        decreased from 94.2 (R_(N)15) to 93.7 (R_(N)15).

U.S. Pat. No. 5,271,758 to Buljan et al. pertains to an alumina-basedcomposite that can include boron carbide and a Ni—Al metallic phase.Example 20 comprises” 8 v/o (Ni,Al), 27.6 v/o B₄C and 64.4 v/o Al₂O₃.U.S. Pat. No. '758 does not appear to specifically recite a hot pressingprocess for Example 20. PCT Patent Publication WO 92/07102 to Buljan etal. published Apr. 30, 1992) appears to be related to U.S. Pat. No.'758. U.S. Pat. No. 5,279,191 to Buljan appears to disclose analumina-based ceramic that may include boron carbide. U.S. Pat. No. '191requires the use of SiC reinforcement and a Ni—Al metal phase.

U.S. Pat. No. 5,162,270 to Ownby et al. pertains to an alumina ceramicthat has boron carbide whisker reinforcement. FIG. 1 appears to showspecific compositions in which the boron carbide whiskers appear tocomprise 0, 5.0, 10.0, 15.0, 20.0 and 30.0 volume percent of thecomposite (the balance alumina). These samples were hot pressed at 1520°C. under a pressure equal to 7500 psi to achieve a density equal togreater than abut 98 percent of theoretical density. The maximumfracture toughness (about 7.1 MPa·m^(0.5)) occurs at 15.0 volume percentboron carbide whiskers. There is a slight decrease in the fracturetoughness (about 7.1 MPa·m^(0.5) to about 7.0 MPa·m^(0.5)) when boroncarbide whisker content exceeds 15 volume percent. U.S. Pat. No.5,398,858 to Dugan et al. mentions the use of boron carbide whiskers toreinforce alumina. The specific application for the ceramic is in aroller guide.

The article by Liu and Ownby entitled “Densification of B₄C WhiskerReinforced Al₂O₃ Matrix Composites”, Proceedings of the First ChinaInternational Conference on High-Performance Ceramics (October, 1998,Beijing) pp. 415-419 pertains to the sintering of boron carbidewhisker-alumina composites. The boron carbide whisker contents were (involume percent): 0, 5, 10, 15, 20, 25, 30, 35 and 40.

The article by Liu et al. entitled “Enhanced Mechanical Properties ofAlumina by Dispersed Titanium Diboride Particulate Inclusions”, JournalAmerican Ceramic Society 74(1) pp. 241-243 (1991) discloses the use oftitanium diboride particles to improve mechanical properties of alumina.FIG. 2 shows the impact of the boron carbide particle content in analumina-based ceramic on the flexural strength wherein the boron carbidecontent ranges from 0 to 20.0 volume percent. Like in the other articlesto Liu et al., the flexural strength appears to level off (or remainsteady) for boron carbide contents that exceed 5.0 volume percent.

U.S. Pat. No. 4,745,091 to Landingham discloses an alumina-based ceramicthat has a nitride modifier (e.g. AlN or Si₃N₄) and dispersionparticles. A listing of the dispersion particles mentions boron carbide.According to the '091 patent, the nitride modifier can range from 0.1 to15.0 weight percent, and the dispersion particles can range between 0.1and 40.0 weight percent. There do not appear to be any actual examplesthat use boron carbide as dispersion particles.

U.S. Pat. No. 6,417,126 B1 to Yang discloses an alumina-based compositewith a boride (e.g., boron carbide) and metal carbide (e.g., siliconcarbide). The examples appear to disclose compositions comprisingalumina, silicon carbide, and boron carbide wherein the boron carbideranges between 0.5 and 5.4 weight percent. U.S. Pat. No. '126 appears todisclose that the principal use of the ceramic is an industrial blastnozzle. U.S. Patent Application Publication U.S. 2002/0195752 A2 to Yangappears to be related to U.S. Pat. No. '126. European Patent 0 208 910to Suzuki et al. appears to disclose the use of boron carbide along withSiC whiskers in an alumina composite.

U.S. Pat. No. 5,164,345 to Rice et al. relates to an alumina-boroncarbide-silicon carbide composite. The end product is the result ofheating silicon dioxide, boron oxide, aluminum and carbon.

The article by Sato et al., “Sintering and Fracture Behavior ofComposites Based on Alumina-Zirconia (Yttria)-Nonoxides”, Journal dePhysique, Colloque C1, Supplement No. 2, Tome 47, February 1986 pp.C1-733 through C1-737 pertains to the sintering of alumina-containingcomposites including an alumina-zirconia-boron carbide composite. Table1 of Sato et al. shows various properties of a 50 volume percentAl₂O₃-40 volume percent ZrO₂ (no yttria)-10 volume B₄C composite, and an80 volume percent Al₂O₃-10 volume percent ZrO₂ (no yttria)-10 volumepercent B₄C composite. Each composite was hot pressed at 1500° C. and 2GPa for a duration of 30 minutes.

The article by Becher entitled “Microstructural Design of ToughenedCeramics” Journal American Ceramic Society 74(2) pp. 255-269 (1991)discusses toughening mechanisms. The principal toughening mechanism iscrack-bridging. Additives include silicon carbide whiskers, tetragonalzirconia and monoclinic zirconia.

U.S. Pat. No. 4,474,728 to Radford and U.S. Pat. No. 4,826,630 toRadford each discloses pellets that comprise alumina and boron carbide.These pellets appear to be useful as neutron absorbers.

While there have been ceramic bodies that comprise alumina and boroncarbide, there remains a need to provide an improved ceramic body thatcontains alumina and boron carbide, and especially alumina and a boroncarbide irregular-shaped phase. There also remains the need to providesuch a ceramic body of alumina and boron carbide that exhibitsproperties that are especially useful for metalcutting. Exemplary ofthese properties are the ability of the ceramic body to maintain itshardness even at higher operating temperatures, especially thosetemperatures associated with higher cutting speeds. Another exemplaryproperty is the ability of the ceramic body to exhibit good chemicalresistance with respect to the workpiece material even at high operatingtemperatures, especially those associated with higher cutting speeds.Each one of these properties by itself, and especially when combinedtogether, provide for a ceramic body that is particularly useful as aceramic cutting insert for applications at higher cutting speeds whereinthere are generated higher operating temperatures. For example, a highercutting speed contemplated by applicants for ductile cast iron could bea speed equal to or greater than about 1500 surface feet per minute(about 457 surface meters per minute), and more preferably, a highercutting speed equal to or greater than about 2000 surface feet perminute (610 surface meters per minute).

SUMMARY OF THE INVENTION

In one form, the invention is a ceramic metalcutting insert for chipforming machining made from a starting powder mixture. The ceramicinsert body comprises a substrate that has a rake surface and a flanksurface wherein the rake surface and the flank surface intersect to forma cutting edge. The substrate comprises between about 15 volume percentand about 35 volume percent of a boron carbide irregular-shaped phaseand at least about 50 volume percent of alumina. The substrate has afracture toughness (K_(IC)) (18.5 Kg Load E&C) equal to or greater thanabout 4.5 MPa·m^(0.5).

In still another form the invention is a process for making a ceramic(wherein the ceramic has a preferred application for cutting toolapplications) comprising the steps of: providing a starting powdermixture that comprises between about 15 volume percent and about 35volume percent of a boron carbide powder, and at least about 50 volumepercent of alumina powder and no more than about 5 volume percent of asintering aid; consolidating the powder mixture at a temperature equalto between about 1400 degrees Centigrade and about 1850 degreesCentigrade to achieve a ceramic with a density equal to or greater than99 percent of the theoretical density.

In yet another form thereof, the invention is a ceramic body thatcomprises between about 15 volume percent and about 35 volume percent ofa boron carbide irregular-shaped phase, and at least about 50 volumepercent of alumina. The substrate has a fracture toughness (K_(IC))(18.5 Kg Load E&C) equal to or greater than about 4.5 MPa·m^(0.5).

In still another form thereof, the invention is a method of machining aworkpiece comprising the steps of: providing a workpiece; providing aceramic cutting insert having a rake surface and a flank surface whereinthe rake surface and the flank surface intersect to form a cutting edgeand the ceramic cutting insert having a substrate that comprises betweenabout 15 volume percent and about 35 volume percent of a boron carbidephase and at least about 50 volume percent alumina and has a fracturetoughness (K_(IC), 18.5 Kg Load E&C) greater than or equal to about 4.5MPa·m^(0.5); causing relative rotational movement between the workpieceand the ceramic cutting insert wherein the surface speed of the relativerotational movement is equal to or greater than about 457 surface metersper minute; and bringing the ceramic cutting insert and the workpieceinto contact with each other so as to remove material from theworkpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

The following is a brief description of the drawings wherein thesedrawings form a part of this patent application:

FIG. 1 is an isometric view of a ceramic cutting insert that embodiesthe invention;

FIG. 2 is a colorized photomicrograph (50 micrometer scale) that showsthe microstructure of the ceramic body of Sample CA340-58 that has astarting composition of about 24.9 volume percent boron carbide powder,about 74.6 volume percent alumina powder, and sintering aid residue from0.5 volume percent of ytterbia (i.e., ytterbium oxide) as a sinteringaid in the starting powder mixture, and in the photomicrograph the lightphase is boron carbide;

FIG. 3. is a photomicrograph (30 micrometer scale) that was made viascanning electromicroscopy (SEM) techniques that shows themicrostructure the ceramic body of Sample CA340-59 that has acomposition of about 25 volume percent boron carbide powder, about 74volume percent alumina powder and sintering aid residue from 1.0 volumepercent ytterbia (ytterbium oxide) as a sintering aid in the startingpowder mixture, and in the photomicrograph the dark phase is the boroncarbide and the light phase is a ytterbium-containing compound;

FIG. 4A is a colorized photograph (at a magnification equal to 30×) ofthe flank surface of a prior art ceramic cutting insert designatedherein as Comparative Insert #1 [KYON 3400] showing the nature of theflank wear on the cutting insert after completion (duration of 6minutes) of the testing set out in Table 5;

FIG. 4B is a colorized photograph (at a magnification equal to 30×) ofthe rake surface of a prior art ceramic cutting insert designated hereinas Comparative Insert #1 [KYON 3400] showing the nature of the craterwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5;

FIG. 5A is a colorized photograph (at a magnification equal to 30×) ofthe flank surface of Sample CA340-58 showing the nature of the flankwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5; and

FIG. 5B is a colorized photograph (at a magnification equal to 30×) ofthe rake surface of Sample CA340-58 showing the nature of the craterwear on the cutting insert after completion (duration of 6 minutes) ofthe testing set out in Table 5.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Referring to the drawings, FIG. 1 shows an indexable ceramic cuttinginsert, i.e., a ceramic body, generally designated as 20. Ceramiccutting insert 20 comprises a substrate 21 that has a rake surface 22and flank surfaces 24 wherein a cutting edge 26 is at the intersectionof the rake surface 22 and the flank surfaces 24. Throughout thisdescription, selected physical properties of the ceramic body (orceramic substrate) are set forth. In regard to the method to determinethese properties, the fracture toughness (K_(IC)) is determined by themethod set forth in Evans & Charles, “Fracture Toughness Determinationby Indentation”, J. American Ceramic Society, Vol. 59, Nos. 7-8, pages371-372 using an 18.5 kilogram load. The Young's Modulus is determinedby ASTM Standard E111-97 Standard Test Method for Young's Modulus,Tangent Modulus and Chord Modulus. The Vicker's micro-hardness isdetermined by ASTM Standard E384-99e1 Standard Test Method forMicroindentation Hardness of Materials using an 18.5 kilogram load.

The ceramic substrate (or ceramic body) 21 of the ceramic cutting insert20 has a composition that comprises primarily alumina and a boroncarbide irregular-shaped phase along with optionally lesser amounts ofadditives such as, for example, sintering aid residue from an additionof sintering aid to the starting powder mixture. The sintering aidtypically comprises in its broader range between about 0.05 volumepercent and about 5 volume percent of the starting powder mixture. Apreferable range for the sintering aid is between about 0.1 volumepercent and about 1.5 volume percent of the starting powder mixture. Amore preferable amount of sintering aid in the starting powder mixtureis about 0.5 volume percent of the starting powder mixture. For theceramic substrate, the content (in volume percent) of the alumina in theceramic is greater than the content (in volume percent) volume percentof the boron carbide irregular-shaped phase in the ceramic. The content(in volume percent) of the boron carbide irregular-shaped phase in theceramic is greater than any other component, except for the alumina inthe ceramic.

Applicants contemplate that other additives may be added in amountseffective to improve the metalcutting performance characteristics (ofthe inventive ceramic cutting inserts) without undergoing a significantreaction with the boron carbide in the ceramic during the densificationof the ceramic body. In this regard, these additives include one or moreof the oxides of zirconium and/or hafnium and silicon carbide whiskers.

In one compositional range, the substrate 21 comprises between about 15volume percent and about 35 volume percent of a boron carbideirregular-shaped phase and at least about 50 volume percent alumina andsintering aid residue. In another compositional range for the ceramicsubstrate, the substrate of the ceramic cutting insert comprises betweenabout 15 volume percent and about 35 volume percent of a boron carbideirregular-shaped phase and between about 65 volume percent and about 85volume percent alumina and sintering aid residue. In yet anothercompositional range for the ceramic substrate, the substrate of theceramic cutting insert comprises between about 20 volume percent andabout 30 volume percent of a boron carbide irregular-shaped phase andbetween about 70 volume percent and about 80 volume percent alumina andsintering aid residue. In still another composition of the ceramicsubstrate, the substrate comprises about 25 volume percent of a boroncarbide irregular-shaped phase and about 75 volume percent alumina andsintering aid residue.

The ceramic substrate (or ceramic body) of the ceramic cutting insertexhibits certain physical properties. These physical properties includeYoung's Modulus (E), fracture toughness (K_(IC)) and Vicker'smicrohardness. Values for these properties are set forth hereinafter.

In one embodiment of the ceramic body, the fracture toughness (K_(IC),18.5 kg load Evans & Charles) is greater than or equal to about 4.5MPa·m^(1/2). In another embodiment of the ceramic body, the fracturetoughness (K_(IC), 18.5 kg load Evans & Charles) is greater than orequal to about 5.0 MPa·m^(1/2). In another embodiment of the ceramicbody, the fracture toughness (K_(IC), 18.5 kg load Evans & Charles) isgreater than or equal to about 5.5 MPa·m^(1/2). In still anotherembodiment of the ceramic body, the fracture toughness (K_(IC), 18.5 kgload Evans & Charles) is greater than or equal to about 6.0 MPa·m^(1/2).

In one embodiment of the ceramic body, the Young's Modulus (ASTMStandard E111-97, Standard Test Method for Young's Modulus, TangentModulus and Chord Modulus) is greater than or equal to about 300 GPa. Inanother embodiment of the ceramic body, the Young's Modulus is greaterthan or equal to about 350 GPa. In yet another embodiment of the ceramicbody, the Young's Modulus is greater than or equal to about 400 GPa.

In one embodiment of the ceramic body, the Vickers microhardness (ASTMStandard E384-99e1, Standard Test Method for Microindentation Hardnessof Materials, 18.5 kg load) is greater than or equal to about 17 GPa. Inanother embodiment of the ceramic body, the Vickers microhardness isgreater than or equal to about 18 GPa. In yet another embodiment of theceramic body, the Vickers microhardness that is greater than or equal toabout 19 GPa.

The ceramic body (after hot pressing) has a density that is greater thanor equal to about 3.6 grams per cubic centimeter. This equates to adensity that is greater than about 99.7 percent of the theoreticaldensity for a composition that comprises about 25 volume percent of aboron carbide irregular-shaped phase and about 75 volume percent aluminaand sintering aid residue.

Although the specific embodiment of FIG. 1 is a ceramic body that takeson the form of a substrate for an uncoated indexable ceramic cuttinginsert, applicants contemplate that the ceramic body has uses other thanas a substrate for a ceramic cutting insert. In this regard, the ceramicbody may have use as a substrate for a coated ceramic cutting insertincluding an indexable ceramic cutting insert. In addition, the ceramicbody may have use as a wear member. Exemplary wear members includenozzles for shot blasting and abrasive water jet applications.

One useful technique to produce the ceramic body is hot pressing.However, applicants contemplate that any consolidation process thatapplies heat and (optionally) pressure for a sufficient duration of timeto achieve the desired density can be an acceptable process. Sinteringis another possible process to produce the ceramic body.

In general, the hot pressing process comprises the following steps thatare described hereinafter. The first step comprises providing a startingpowder mixture wherein the starting powder mixture has a compositionthat falls within one of the compositional ranges contemplated by theinvention as set forth in this patent application. The basic componentsof the starting powder mixture are a majority content of alumina powder,a minority content of boron carbide powder, and a minor content (e.g.,about 0.5 volume percent) of a sintering aid or in some cases, anotheradditive as mentioned above (e.g., the oxides of zirconium and/orhafnium and/or silicon carbide whiskers). The sintering aid can compriseone or more materials that are suitable for use as a sintering aid forceramics. Exemplary sintering aids include oxides such as, for example,yttrium oxide, yttrium aluminum garnet (YAG), ytterbium oxide, lanthanumoxide and chromium oxide.

The second step in the hot pressing process comprises hot pressing thestarting powder mixture under pressure and heat to form the ceramicbody. The hot pressing conditions are generally defined by the hotpressing temperature, hot pressing pressure and the duration of the hotpressing process. In regard to the hot pressing parameters, the hotpressing temperature has one range that is between about 1400 degreesCentigrade and about 1850 degrees Centigrade, as well as a narrowerrange that is between about 1400 degrees Centigrade and about 1700degrees Centigrade. The hot pressing pressure has a range that isbetween about 20 MPa and about 50 MPa. The hot pressing duration has arange that is between about 20 minutes and about 90 minutes. The hotpressing process may occur under a vacuum (i.e., a pressure equal to orless than about 100 micrometers of mercury) or an inert gas atmosphere.

The hot pressing process produces a ceramic body that exhibits physicalproperties that include the fracture toughness (K_(IC), 18.5 Kg LoadE&C), the Young's Modulus and the Vickers hardness. The typical valuesof these properties have been set forth in this patent application.

Tests to determine selected physical properties (e.g., Young's Modulus,Vickers microhardness and fracture toughness (K_(IC))), as well asmetalcutting performance, were conducted on specific examples (orsamples) of the ceramic body to compare the performance of specificexamples of the alumina-boron carbide irregular-shaped phase ceramiccutting inserts against the performance of standard cutting inserts. Thesteps of the process employed to make the alumina-boron carbideirregular-shaped phase ceramic cutting inserts that were the subjects ofthe tests to determine the physical properties and the metalcuttingperformance are set forth below.

In regard to the specific powders used in the samples, for most of theexamples, the boron carbide powder (which has a blocky-angular shape)was sold by Electro Abrasives (having a place of business at 701 WilletRoad, Buffalo N.Y. 14218) under the designation F800 wherein the powderhas the following properties: median particle size equal to about 15micrometers, a surfaces area (as measured by BET) equal to 1.5 m²/gram,an oxygen content equal to 0.64 weight percent, the total boroncontent=77.5 weight %, the total carbon content=21.5 weight %, iron=0.2weight %, and the total B+C content=98 weight %. Additional informationabout the boron carbide powders sold by Electro-Abrasives is availablethrough the website: http:/www.electroabrasives.com/b4C.html.

The other kind of boron carbide powder, which is designated as “HP” inTable 1, was sold under the designation Grade HP by H.C. Starck, Inc. 45Industrial Place, Newton, Mass. 02461. The Grade HP boron carbide powderhas the following chemical characteristics: a B:C ratio=3.8-3.9, minimumof 21.8 weight % carbon, maximum of 0.7 weight nitrogen, maximum of 1.0weight % oxygen, maximum of 0.05 weight % iron, maximum of 0.15 weight %of silicon, maximum of 0.05 weight percent aluminum, and a maximum of0.5 weight % of other components. The Grade HP boron carbide powder hasthe following physical characteristics: specific surface area (TRISTAR3000 by BET per ASTM D 3663)=6 to 9 m²/gram; green density (10³kg/cm²)=1.5 to 1.7 g/cm²; particle size distribution with typical values(MASTERSIZER by Laser Light Diffraction per ASTM B 822, deglomerationwith high energy ultrasonic before analysis)=D90=6.5 micrometers,D50=2.5 micrometers, D10=0.4 micrometers. The above chemicalcharacteristics and physical characteristics are available from thewebsite http:/www.hcstarck.com and are set forth in the H.C. Starck DataSheet Number PD-4012.

The alumina powder was sold by Baikowski International (having a placeof business at 352 Westinghouse Blvd., Charlotte, N.C. 28273) under thedesignation SM8 wherein the powder has the following properties: BETspecific surface area equal to 10 m²/gram, an alpha crystal structure,an alpha crystallite size/XRD=50 nanometers (nm), an ultimate particlesize/TEM=400 nanometers (nm), and a purity greater than 99.99 percent.The agglomerate size distribution/sedigraph is: D20=0.2 micrometers;D50=0.3 micrometers; D90=0.7 micrometers. This information about the SM8alumina powder is available at the website: http:/www.baikowski.com.

Another kind of alumina powder was used for at least one other example,and that was alumina powder sold under the designation “HPA-0.5” bySasol North America, Inc., Ceralox Division, having a place of businessat 7800 South Kolb Road, Tucson, Ariz. 85706. The HPA-0.5 alumina powderhas the following properties: a purity equal to 99.99 weight percent; asurface area=9.0 m²/gram; a green density equal to 2.19 grams/cubiccentimeter; and a particle size distribution of D90=1.2 micrometers,D50=0.4 micrometers and D10=0.2 micrometers. Additional informationabout the properties and the HPA-0.5 alumina powder can be found at thewebsite: http:/www.ceralox.com/Documents/PDFfiles/TDS-ceramicpowders.pdf.

For the sintering aids, the yttrium oxide powder was sold by MolycorpInc. (having a place of business at 67750 Bailey Road, Mountain Pass,Calif. 97366) wherein the powder had the following properties: surfacearea equal to 1.8 m²/gram, a particle size (Microtrac d50) equal to 3-6micrometers (μm), and a purity equal to greater than 99.0 percent. Theytterbium oxide powder was sold by MolyCorp Inc. under the designationYb₂O₃ 99% and has the following properties: particle size (FAPS) 3 μmmax. and a purity greater than 99 percent. The lanthanum oxide powderwas sold by MolyCorp Inc. under the designation La₂O₃ 99.99% and has thefollowing properties: particle size (FAPS) 5-10 μm maximum and a puritygreater than 99.9 percent. The YAG powder was sold by Cerac Inc. (havinga place of business at P.O. Box 1178, Milwaukee, Wis. 53201-1178) underthe designation Y-2000 and has the following properties: formula isY₃Al₅O₁₂, average particle size −325 mesh and a purity greater than 99.9percent.

To produce the starting powder mixture, the mixture of the startingpowders of alumina and boron carbide and the sintering aid was subjectedto ball-milling using high purity alumina cycloids for a duration equalto about 36 hours in alcohol. After completion of the ball-milling, thepowder mixture was dried.

For all of the examples, each one of the starting powder mixture was hotpressed to form a ceramic body. For all of the examples, unlessindicated to the contrary, the hot pressing was done using a graphitedie and graphite rams, and the hot pressing parameters were atemperature equal to about 1650 degrees Centigrade for a duration ofabout 1 hour under a pressure of about 35 MPa. The ceramic body was thenfinished ground to form the geometries of the alumina-boron carbideirregular-shaped phase ceramic cutting inserts used in the metalcuttingtests set forth below. The geometries of the ceramic cutting inserts areset forth in each of the tables.

Table 1 below sets forth the starting powder compositions for a numberof the compositions that are contained in the Tables set forthhereinafter. TABLE 1 Compositions of the Starting Powder Mixtures forthe Samples of the Ceramic Cutting Inserts as Reported in the TablesBoron Carbide Alumina (volume Particles (volume Additive (volume Samplepercent) percent) percent) CA340-30 74.6 (SM8) 24.9 (F800) 0.5 yttriaCA340-62 74.6 (SM8) 24.9 (HP) 0.5 yttria CA340-57 74.6 (SM8) 24.9 (F800)0.5 YAG CA340-58 74.6 (SM8) 24.9 (F800) 0.5 ytterbia CA340-63 74.6 (SM8)24.9 (F800) 0.5 lanthanum oxide CA340-67 74.6 (SM8) 24.9 (F800) 0.5ytterbia AA301-013 74.5 (HPA-0.5) 25 (F800) 0.5 yttria CA340-59 74.25(SM8) 24.75 (F800) 1.0 ytterbia

Table 1 sets forth herein above presents the compositions of the samplesof the ceramic cutting inserts that were subjected to metalcuttingtests, and testing for physical properties, wherein the test results areset forth in the Tables in this patent application. The compositions arereported in volume percent of the starting powder mixture. Except forSample AA301-013, which used the Ceralox HPA-0.5 alumina powder, all ofthese samples used the SM8 alumina powder described earlier herein. Thedesignation “F800” for the boron carbide powder means that the boroncarbide powder was the F800 boron carbide powder from Electro-Abrasivesdescribed earlier herein. The designation “HP” for the boron carbidepowder means the boron carbide powder was the HP boron carbide powderfrom H.C. Starck described earlier.

Table 2 set forth herein presents the results of measuring the Young'sModulus according to ASTM Standard E111-97 wherein the results arereported in gigapascals (GPa), the Vicker's Microhardness (18.5 kg load)according to ASTM Standard E384-99e1 wherein the results are reported ingigapascals (GPa), and the fracture toughness (K_(IC)) as measuredaccording to Evans & Charles using an 18.5 kg load and reported inMPa·m^(1/2). TABLE 2 Room Temperature Properties for SelectedCompositions Vickers Young's Modulus - Microharadness Fracture ToughnessComposition E (GPa) (VHN (GPa) (K_(IC)) (MPa · m^(1/2)) AA301-013 39218.9 — CA340-30 395 18.7 5.46 CA340-57 414 18.3 5.00 CA340-58 400 18.14.94 CA340-62 395 18.7 4.82 CA340-63 399 18.1 5.09

FIG. 2 is a colorized photomicrograph that shows the microstructure ofthe ceramic body of Sample CA340-58 that has a starting composition setforth in Table 1 and the Room Temperature properties set forth in Table2. The light phase in the photomicrograph is the boron carbide phase.One can see that the boron carbide phase does present an irregular shapeand the distribution of the boron carbide phase is relatively uniformthroughout the microstructure. The darker goldish phase is the alumina.

FIG. 3. is a photomicrograph that was made via scanningelectromicroscopy (SEM) techniques that shows the microstructure of theceramic body of Sample CA340-59 that has a composition as set forth inTable 1. In this photomicrograph, the dark phase is the boron carbidephase. One can see that the boron carbide phase presents an irregularshape. It can be seen that the boron carbide phase appears to be about 6micrometers or less, and more typically about 3 micrometers or less, inits major dimension. The light phase is a ytterbium-containing compoundwhich is sintering aid residue. The gray phase is the alumina.

Table 3 set out below reports the results from turning a round clean barof ductile cast iron (80-55-06) wherein these results show a comparisonbetween a ceramic cutting insert of the invention (designated as SampleAA 301-013 wherein the starting powder composition is set forth inTable 1) and a number of comparative cutting inserts. The inventiveceramic cutting insert (Sample AA 301-013) has a composition of about 25volume percent of a boron carbide irregular-shaped phase, about 74.5volume percent alumina, and sintering aid residue from about 0.5 volumepercent of yttria sintering aid in the starting powder mixture.Comparative Cutting Insert K090 has a composition that comprises about30 volume percent titanium carbide and the balance (70 volume percent)alumina. Comparative Insert Kyon1615 has a composition that comprises 75volume percent alumina-25 volume percent titanium carbonitride. Thecutting insert that carries the designation LA 17/02 in the tables has acomposition of 75 volume percent alumina-25 volume percent titaniumcarbonitride. The cutting insert that carries the designation CB347-216has a composition of 42 volume percent alumina-43 volume percenttitanium carbonitride-15 volume percent silicon carbide whiskers. Thecutting insert that has the designation alumina-SiC whisker is a cuttinginsert that has a composition of about 15 volume percent silicon carbidewhiskers and the balance (about 85 volume percent) alumina. ComparativeCutting Insert KYON 3400 is a chemical vapor deposition (CVD) coatedsilicon nitride substrate.

This metalcutting test comprised the turning of a round clean bar ofductile Cast Iron 80-55-06. The turning parameters were: a speed of 1500surface feet per minute (457 surface meters per minute), a feed of 0.015inches (0.38 millimeters) per revolution, and a depth of cut of 0.100inches (2.54 millimeters) d.o.c. The metalcutting was dry, i.e., nocoolant. The geometries of the cutting inserts are set forth in Table 3below wherein the lead angle for all of the cutting inserts was 15degrees. The geometries are identified according to the AmericanNational Standard for Cutting Tools-Indexable Inserts-IdentificationSystem, ANSI B212.4-1986. The failure criteria for this test is asfollows: Flank Wear (UNIF)=0.020 inches (0.508 mm); Flank Wear(MAX)=0.020 inches (0.508 mm); Nose Wear=0.020 inches (0.508 mm); andTrailing edge wear=0.020 inches (0.508 mm). TABLE 3 Metalcutting Test(TR9722A) Results for Turning at a Speed of 1500 Surface Feet Per Minuteof Ductile Cast Iron 80-55-06 Using Different Cutting Inserts Rep. 1Rep. 2 Rep. 3 Tool Tool Tool Tool Mean T.L. Material Geometry Life LifeLife (minutes) AA301-013 SNG433T0425 6.6 3.4 5.1 5.0 KO90 SNG453T08200.8 0.3 — 0.4 KYON 1615 SNG453T0820 0.3 0.7 — 0.3 LA 17/02 SNG454T08251.3 3.0 — 1.4 CB347-216 SNG453T0425 2.0 3.3 3.0 2.8 Alumina- SNG453T08204.7 4.1 2.0 3.6 SiC Whisker KYON 3400 SNG453T0820 — 3.1 3.1 3.1

As can be seen from the insert designations set forth in the secondcolumn from the left side in Table 3, the sizes, geometries and edgepreparations for some of the cutting inserts were different. Based uponapplicants' experience, and later verified by additional tests, thesedifferences in sizes, geometries and edge preparations between thecutting inserts that were tested did not have a significant impact uponthe test results. Hence, applicants believe that the test resultsreported in Table 3 comprise a fair comparison between cutting insertsof the invention and the other cutting inserts.

These test results show that the ceramic cutting insert of the invention(Sample AA301-013) exhibited superior tool life when cutting at a speedequal to 1500 sfm (457 smm) as compared to a number of other prior artceramic cutting inserts. More specifically, the invention showedexcellent crater wear and nose wear resistance, as compared with thecomparative cutting inserts. The crater wear and nose wear resistanceare the key factors for controlling the tool life when cutting (e.g.,turning) at a high speed (e.g., a speed equal to 1500 sfm (457 smm)). Inother words, better crater wear and nose wear properties result in alonger tool life for a ceramic cutting insert when cutting (e.g.,turning) at a high speed (e.g., a speed equal to 1500 sfm (457 smm)).

Table 4 set out below reports the results from turning a round clean barof ductile cast iron (80-55-06) wherein the results show a comparisonbetween a ceramic cutting insert of the invention (designated as SampleCA340-67) and a comparative cutting insert (Kyon3400). Sample CA340-67has a composition of about 24.9 volume percent of the boron carbideirregular-shaped phase, about 74.6 volume percent alumina, and sinteringaid (ytterbia) residue that is from a starting powder content ofsintering aid equal to about 0.5 volume percent. The turning parameterswere: a speed of 2000 surface feet per minute (609.6 surface meters perminute), a feed of 0.015 inches (0.38 millimeters) per revolution, and adepth of cut of 0.100 inches (2.54 millimeters) d.o.c. The metalcuttingwas dry, i.e., no coolant. The geometries of the cutting inserts are setforth in Table 4 below wherein the lead angle for all of the cuttinginserts was 15 degrees. The failure criteria for this test is asfollows: Flank Wear (UNIF)=0.020 inches (0.508 mm); Flank Wear(MAX)=0.020 inches (0.508 mm); Nose Wear=0.020 inches (0.508 mm); andTrailing Edge Wear=0.020 inches (0.508 mm). TABLE 4 Metalcutting Test(T11338) Results (tool life in minutes) for Turning at a Speed of 2000Surface Feet Per Minute of Ductile Cast Iron 80-55-06 Tool Life MeanTool Tool Material Geometry Rep 1 Rep 2 Life (minutes) Kyon 3400SNGN433T0820 2.5 3.9 3.2 CA340-67 SNGN433T0820 8.0 7.7 7.8The inventive ceramic cutting insert (Sample CA340-67) significantlyoutperformed the cutting insert of the comparative grade (KYON 3400).Applicants believe that this improvement in performance was due to thesuperior chemical wear resistance provided by the inventive ceramiccutting inserts at higher cutting speeds (e.g., 2000 sfm (610 smm))wherein at such higher cutting speeds, the chemical wear exerts greatinfluence over (i.e., dominates) the tool life.

Additional metalcutting test results demonstrate the performance ofspecific samples of the ceramic cutting insert of the invention. Thesetest results are set forth below.

Except for the speed, each one of the tests referred to in Tables 5 and6 was conducted at the following parameters: the feed equal to 0.015inches (0.381 millimeters); the Depth of cut (DOC) equal to 0.100 inches(2.54 mm); and the coolant: dry. The speed for the tests reported inTable 5 was 1500 feet per minute (457 meters per minute) and the speedfor the tests reported in Table 6 was 2000 feet per minute (610 metersper minute). For each of the tests, the geometry of the cutting insertwas a SNG433T0820 style of cutting insert that had a negative 5 degreelead angle. The workpiece material was a round clean bar of ductile castiron (80-55-06). The failure criteria for these tests set forth inTables 5 and 6 were as follows: Flank Wear (UNIF)=0.020 inches (0.508mm); Flank Wear (MAX)=0.020 inches (0.508 mm); Nose Wear=0.020 inches(0.508 mm); and Trailing Edge Wear=0.020 inches (0.508 mm). TABLE 5T10828_Turning DCI 80-55-06 SNG-433T0820/1500 sfm/.015 ipr/.1″ doc/dryAverage Insert Wear After 6 min. Turning Mean Tool Insert # FW MW NW TWLife (min) 1. KYON 3400 0.0124 0.0170 0.0190 0.0146 5.6 2. CA340-300.0143 0.0173 0.0165 0.0153 6.8 3. CA340-62 0.0139 0.0205 0.0154 0.01385.0 4. CA340-57 0.0130 0.0161 0.0148 0.0152 7.2 5. CA340-58 0.01290.0150 0.0152 0.0154 7.0 6. CA340-63 0.0125 0.0153 0.0153 0.0153 7.4In Table 5 above, the designations “FW” means average flank wearreported in inches, “MW” means average maximum flank wear reported ininches, “NW” means average nose wear reported in inches, and “TW” meansaverage trailing edge wear reported in inches. The mean tool life isreported in minutes.

Referring to the test results presented in Table 5 above, it is apparentthat, for the most part, the ceramic cutting inserts of the inventionoutperformed the KYON 3400 ceramic cutting insert. The KYON 3400 cuttinginsert is a commercial cutting insert that is well-accepted for the usein the turning of ductile cast iron. More specifically, except forInsert No. 3 (Sample CA340-62) which had a mean tool life equal to about89.2 percent of the mean tool life of the KYON 3400 cutting insert, allof the ceramic cutting inserts demonstrated an improved mean tool life.In this regard, Insert No. 2 (Sample CA340-30) had a mean tool lifeequal to about 121.4 percent of the mean tool life of the KYON 3400cutting insert, Insert No. 4 (Sample CA340-57) had a mean tool lifeequal to about 128.6 percent of the mean tool life of the KYON 3400cutting insert, Insert No. 5 (Sample CA340-58) had a mean tool lifeequal to 125 percent of the mean tool life of the KYON 3400 cuttinginsert, and Insert No. 6 (Sample CA340-63), which used the lanthanumoxide sintering aid, had a mean tool life equal to about 132.1 percentof the mean tool life of the KYON 3400 cutting insert.

Based upon a comparison of the test results for Insert No. 2 and InsertNo. 3, it appears that the ceramic cutting insert that used the F800boron carbide (from Electro-Abrasives) had better results (i.e., alonger mean tool life) than the ceramic cutting insert that used the HPboron carbide (from H.C. Starck).

A comparison of the test results for an alumina-boron carbideirregular-shaped phase ceramic cutting insert using yttria as thesintering aid (i.e., Insert No. 2) against the alumina-boron carbideirregular-shaped phase ceramic cutting inserts using other sinteringaids shows that these other sintering aids (i.e., YAG, ytterbium andLa₂O₃) provided for improved results in the form of a longer mean toollife.

FIGS. 4A and 5A illustrate the comparison of the flank wear propertiesbetween a Comparative Insert #1 and a cutting insert of the invention(Sample CA340-58). More specifically, FIG. 4A is a colorized photograph(at a magnification equal to 30×) of the flank surface of theComparative Insert #1 [KYON 3400] showing the nature of the flank wearon the cutting insert after completion (duration of 6 minutes) of thetesting set out in Table 5. FIG. 5A is a colorized photograph (at amagnification equal to 30×) of the flank surface of Sample CA340-58showing the nature of the flank wear on the cutting insert aftercompletion (duration of 6 minutes) of the testing set out in Table 5. Itis apparent from an examination of the cutting inserts shown in FIGS. 4Aand 5A, that the inventive cutting insert experienced less flank wearand a more uniform flank wear than did the prior art comparative cuttinginsert (Comparative Insert #1).

FIGS. 4B and 5B show a comparison of the crater wear properties betweena Comparative Insert #1 and a cutting insert of the invention (SampleCA340-58). More specifically, FIG. 4B is a colorized photograph (at amagnification equal to 30×) of the rake surface of the ComparativeInsert #1 [KYON 3400] showing the nature of the crater wear on thecutting insert after completion (duration of 6 minutes) of the testingset out in Table 5. FIG. 5B is a colorized photograph (at amagnification equal to 30×) of the rake surface of Sample CA340-58showing the nature of the crater wear on the cutting insert aftercompletion (duration of 6 minutes) of the testing set out in Table 5. Itis apparent from an examination of FIGS. 4B and 5B that the inventivecutting insert (Sample CA340-58) experienced less crater wear than didthe prior art cutting insert (Comparative Insert #1).

Metalcutting tests also show that the inventive ceramic cutting insertsexhibit better performance (i.e., tool life) at even higher cuttingspeeds, e.g., on the order of 2000 sfm (610 smm). In this regard, Table6 below sets forth the results for the turning of ductile cast iron atthe parameters (including a speed equal to 2000 sfm (610 smm)) set forthby cutting inserts of the geometry (SNG-433T0820) presented in Table 6.As seen by the results presented in Table 6, the inventive cuttinginserts exhibit a much greater mean tool life than the KYON 3400 cuttinginsert. More specifically, the Insert No. 2 (Sample CA340-30), which isan alumina-boron carbide irregular-shaped phase ceramic cutting insertthat used the F800 boron carbide, had a mean tool life equal to 244percent of the mean tool life of the KYON 3400 ceramic cutting insert.Insert No. 5 (Sample CA340-58), which is an alumina-boron carbideirregular-shaped phase ceramic cutting insert that used ytterbia as thesintering aid, had a mean tool life that was 292 percent of the meantool life of the KYON 3400 ceramic cutting insert. TABLE 6 Mean ToolLife Reported in Minutes T10919_Turning DCI 80-55-06 SNG-433T0820/2000sfm/.015 ipr/.1″ doc/dry Insert # Test No. 1 Test No. 2 Mean Tool Life(min) 1. KYON 3400 3.0 1.9 2.5 2. CA340-30 7.4 4.7 6.1 5. CA340-58 7.57.0 7.3 6. CA340-63 4.5 6.6 5.5

When increasing the turning speeds from 1500 sfm (457 m/min) to 2000 sfm(610 m/min), as shown in metal cutting test T10919 (Table 6 below), theperformance of KY3400 degraded considerably while the influence ofspeeds on the performance of alumina-boron carbide composites was not sosignificant. The cutting insert (Sample CA340-63) that used lanthanumoxide as the sintering aid had a mean tool life that was over twice aslong (i.e., 5.5 minutes vs. 2.5 minutes) as the mean tool life for theKYON 3400 cutting insert.

Overall, it is apparent that applicants have invented a new and usefulceramic body that comprises alumina and a boron carbide irregular-shapedphase, and optionally, the sintering aid residue from a sintering aidcontained in the starting powder mixture. The ceramic body can be usedas a wear member, as well as an uncoated ceramic cutting insert or acoated ceramic cutting insert.

When used as a ceramic cutting insert, the ceramic substrate hasmaintained its wear resistance even at higher operating temperatures,especially those temperatures associated with higher cutting speeds(e.g., a speed equal to or greater than about 1500 sfm (457 smm), or ateven higher cutting speeds equal to or greater than about 2000 sfm (610smm)). The ceramic substrate has also been able to exhibit good chemicalresistance with respect to the workpiece material.

These improved properties demonstrate that overall the alumina-boroncarbide irregular-shaped phase ceramic cutting inserts of the inventionoutperform (as measured by mean tool life in the turning of ductile castiron) the conventional commercial ceramic cutting insert (a CVD coatedsilicon nitride cutting insert). This is especially true at highercutting speeds in the order of 2000 sfm (610 smm). This is also markedlyapparent when the sintering aid comprises a material like YAG or Yb₂O₃or La₂O₃ or Y₂O₃.

All patents, patent applications, articles and other documentsidentified herein are hereby incorporated by reference herein. Otherembodiments of the invention may be apparent to those skilled in the artfrom a consideration of the specification or the practice of theinvention disclosed herein. It is intended that the specification andany examples set forth herein be considered as illustrative only, withthe true spirit and scope of the invention being indicated by thefollowing claims.

1. A ceramic metalcutting insert for chip forming machining made from astarting powder mixture, the ceramic insert body comprising: a substratehaving a rake surface and a flank surface wherein the rake surface andthe flank surface intersect to form a cutting edge; the substratecomprising between about 15 volume percent and about 35 volume percentof a boron carbide irregular-shaped phase and at least about 50 volumepercent alumina; and the substrate having a fracture toughness (K_(IC),18.5 Kg Load E&C) greater than or equal to about 4.5 MPa·m^(0.5).
 2. Theceramic metalcutting insert according to claim 1 wherein the substratehas a fracture toughness (K_(IC), 18.5 Kg Load E&C) greater than orequal to about 5.5 MPa·m^(0.5).
 3. The ceramic metalcutting insertaccording to claim 1 wherein the substrate has a Young's Modulus equalto or greater than about 300 GPa.
 4. The ceramic metalcutting insertaccording to claim 1 wherein the substrate has a Vicker's micro-hardnessequal to or greater than about 17 GPa.
 5. The ceramic metalcuttinginsert according to claim 1 wherein the substrate comprises betweenabout 15 volume percent and about 35 volume percent of the boron carbideirregular-shaped phase and between about 65 volume percent and about 85volume percent alumina.
 6. The ceramic metalcutting insert according toclaim 1 wherein the substrate comprises between about 20 volume percentand about 30 volume percent of the boron carbide irregular-shaped phaseand between about 70 volume percent and about 80 volume percent alumina.7. The ceramic metalcutting insert according to claim 1 wherein thesubstrate comprises about 25 volume percent of the boron carbideirregular-shaped phase and about 75 volume percent alumina.
 8. Theceramic metalcutting insert according to claim 1 wherein the substratefurther comprises residue from a sintering aid in the starting powdermixture and the sintering aid is selected from the group comprisingyttrium oxide, ytterbium oxide, yttrium aluminum garnet, lanthanumoxide, and chromium oxide.
 9. The ceramic metalcutting insert accordingto claim 1 wherein the substrate further includes the constituents fromone or more of the following additives in the starting powder mixture:the oxides of hafnium and/or zirconium, and silicon carbide whiskers.10. The ceramic metalcutting insert according to claim 1 furtherincluding a refractory coating on the substrate.
 11. A process formaking a ceramic body comprising the steps of: providing a startingpowder mixture, the starting powder mixture comprises between about 15volume percent and about 35 volume percent boron carbide powder and atleast about 50 volume percent alumina powder and no more than about 5volume percent of a sintering aid; and consolidating the powder mixtureat a temperature equal to between about 1400 degrees Centigrade and 1850degrees Centigrade to achieve a ceramic with a density equal to greaterthan 99 percent of theoretical density.
 12. The process according toclaim 11 wherein the consolidating conditions further comprise a hotpressing pressure equal to between about 30 MPa and about 40 MPa. 13.The process according to claim 11 wherein the consolidating conditionsfurther comprise a hot pressing duration equal to between about 30minutes and about 90 minutes.
 14. The process according to claim 11wherein the ceramic body has a fracture toughness (K_(IC), 18.5 Kg LoadE&C) greater than or equal to about 4.5 MPa·m^(0.5).
 15. The processaccording to claim 11 wherein the ceramic body has a Young's Modulusgreater than or equal to about 300 GPa.
 16. The process according toclaim 11 wherein: the starting powder mixture comprises between about 20volume percent and about 30 volume percent of the boron carbide powderand between about 70 volume percent and about 80 volume percent of thealumina powder.
 17. The process according to claim 11 further includingthe step of applying a coating to the ceramic body.
 18. A ceramic bodycomprising: between about 15 volume percent and about 35 volume percentof a boron carbide irregular-shaped phase, and at least about 50 volumepercent alumina, and the ceramic body has a fracture toughness (K_(IC),18.5 Kg Load E&C) greater than or equal to about 4.5 MPa·m^(0.5). 19.The ceramic body according to claim 18 wherein the body further includesthe residue from a sintering aid.
 20. A method of machining a workpiececomprising the steps of: providing a workpiece; providing a ceramiccutting insert having a rake surface and a flank surface wherein therake surface and the flank surface intersect to form a cutting edge andthe ceramic cutting insert having a substrate that comprises betweenabout 15 volume percent and about 35 volume percent of a boron carbidephase and at least about 50 volume percent alumina and has a fracturetoughness (K_(IC), 18.5 Kg Load E&C) greater than or equal to about 4.5MPa·m^(0.5); causing relative rotational movement between the workpieceand the ceramic cutting insert wherein the surface speed of the relativerotational movement is equal to or greater than about 457 surface metersper minute; and bringing the ceramic cutting insert and the workpieceinto contact with each other so as to remove material from theworkpiece.
 21. The method of claim 20 wherein the surface speed of therelative rotational movement is equal to or greater than about 610surface meters per minute.
 22. The method of claim 20 wherein theworkpiece material is cast iron.
 23. The method of claim 20 wherein theworkpiece material is ductile cast iron.